We describe the structural and ultrastructural sequence of events that occurs during the acrosome reaction process in Cancer setosus Molina, 1782. No change is observed in spermatophores extracted from the male vas deferens and incubated in different concentrations of seawater; the spermatozoa are not released nor do they experience the acrosome reaction. Only spermatozoa from the masses in seminal receptacles of females undergo acrosome reaction. The acrosome reaction begins with an elongation of the acrosomal vesicle after the spermatozoa are incubated for 15 to 20 min in seawater. During this elongation, the material in the perforatorial chamber is extruded and spread on the outer side walls of the acrosome forming a ring-shaped opening at the anterior region of the acrosome. The ring-shaped material moves to the posterior region, thus increasing the diameter of the anterior opening of the acrosome. In the following stages, an acrosomal filament is extruded and extends beyond the anterior end of the acrosome. The distal end is enlarged and then divided into three thin terminal extensions.